School buses have for many years been equipped with a stop sign mechanism under the control of the bus driver, the mechanism acting to swing out the stop sign from its normally retracted position against the side of the bus to an outstretched position to provide a signal alerting drivers of nearby vehicles that children are entering or alighting the bus. U.S. Pat. Nos. 2,384,689 and 3,094,683 are illustrative of manually-operated school bus sign devices, while U.S. Pat. No. 2,252,529 discloses a hydraulically-operated school bus sign.
It is also known to provide motor-operated school bus signs, such apparatus being illustrated in U.S. Pat. Nos. 2,281,717 and 4,138,668. Of greatest prior art interest in this regard is the 1982 U.S. Pat. No. 4,339,744 to Latta, Jr. In this patent, the stop sign mounted on the side of the bus is operated by a unidirectional DC motor and a linkage associated with the sign. This linkage in conjunction with limit switches acts to deploy and retract the stop sign.
Also of prior art interest is the U.S. Pat. No. 4,697,541 to Wicker, wherein a school bus is provided with a crossing arm attached to the front bumpers of the bus for movement between a retracted position alongside the bumper to an extended position projecting outwardly from the bumper to protect children walking in front of the bus.
In U.S. Pat. No. 4,766,413, there is disclosed a system including control circuit and a hinged stop sign for a school bus. That system includes an unidirectional DC motor for driving the stop sign through an eccentric drive and a link arm having associated therewith two normally closed limit switches. In the course of a motor operating cycle, the stop sign swings out to its fully outstretched position, at which point one limit switch is caused to open, and then returns to its retracted position, at which point the other limit switch opens. The limit switches are included in a control circuit which supplies power to the motor.
The control circuit is responsive to a door switch associated with the bus door so that when the door is opened, the door switch is then closed, and power is supplied to the motor through one closed limit switch until the sign reaches its outstretched position, at which point this limit switch opens to cut off the motor. When the door is closed and the door switch is then open, power is supplied to the motor through the other closed limit switch until the sign reaches its retracted position, at which point this limit switch opens to cut off the motor. Thus, the operation of the sign is coordinated with that of the bus door to provide a stop signal when the need therefor arises. Similar limit or microswitch arrangements are included in the control circuits disclosed in the above-identified Latta, Jr., patent.
The use of limit switches in a control system for a school bus safety device has certain practical drawbacks. Such switches are highly sensitive and therefore may respond falsely to vibratory forces. Thus, where the microswitches are associated with a link arm coupled to a stop sign, should the sign vibrate, these vibrations will be transferred to the link arm and cause improper actuation of the switches.
Also, in some safety device control circuits using limit switches, the switches are interposed between the motor and the positive or high side of the power supply. Should either of the limit switches become grounded as a result of a circuit defect, or because of water seepage into the control box mounted on the exterior of the bus, the resultant heavy current flow may overheat the circuit and give rise to a fire. This condition is scarcely conducive to the safety of the children carried by the school bus.
In U.S. Pat. Application Ser. No. 345,188, filed on May 1, 1989, entitled Control System For School Bus Safety Device, there is disclosed a system which overcomes those disadvantages of the prior art. That system comprises a control switch which is caused to close when the bus door is opened and to open when the bus door is closed. Associated with the control switch is a control circuit for connecting a DC power supply to a DC motor operatively coupled through an eccentric drive and a link arm to the safety device, e.g., stop sign, whereby in the course of a single cycle of motor rotation, during one half of the cycle the device is caused to swing out to its fully outstretched position, and during the other half, the device is caused to return to its fully retracted position. The control circuit basically comprises a commutator formed by a conductive rotor mounted on the motor shaft for rotation therewith and having first and second electrically conductive brushes. The brushes act to engage the rotor at diametrically-opposed positions. The rotor has an insulating element at a particular point on its periphery (called the "zero" position).
The motor is connected to the power supply through the control switch and the commutator, such that when the door is opened and the control switch is then closed, the motor is energized through the first brush to swing the device to its fully outstretched position. At this point the first brush is disconnected by the element to shut off the motor. When the door is closed and the control switch is then open, the motor is then energized through the second brush. This action returns the device to its fully retracted position, at which point the second brush is disconnected by the element to shut off the motor.
While the system disclosed in the foregoing patent application is suitable for its intended purposes, it has been found that in some applications, due to prevailing weather conditions, e.g., in cold-damp climates, long term reliability of the system may suffer somewhat. This is caused by surface contamination of the electrically conductive motor.